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Newtype deriving doesn't work with type families

    • Closed Issue created by rl@cse.unsw.edu.au @trac-rl

    This assumes -XTypeFamiles -XGeneralizedNewtypeDeriving. Example:

    class C a where
  type T a
  foo :: a -> T a

instance C Int where
  type T Int = Int
  foo = id

newtype N = N Int deriving(C)

    This happily produces an instance C N but no type instance T N. It should either (preferably) generate

    type instance T N = Int

    or fail. The example also compiles if T is a data family (the Int instance needs to be change accordingly). It should probably fail in this case.

    BTW, this also compiles fine, with rather dramatic consequences:

    type family T a
class C a where
  foo :: a -> T a

type instance T Int = Int
instance C Int where
  foo = id

type instance T N = Double
newtype N = N Int deriving(C)

    I guess this last example is the same bug as #1496 (closed). I wonder if the deriving clause could generate something like:

    instance T Int ~ T N => C Int
    Edited by Simon Peyton Jones
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